The photochemical inactivation of enzymes is being investigated with flash photolysis and related techniques with emphasis on spectroscopic identification of the short-lived initial products and the relationship to the subsequent changes in biological activity. Current work on ultraviolet irradiation is being carried out with enzymes of known structure including bovine trypsin, papain, and bovine carbonic anhydrase. The enzyme studies are accompanied by continuing investigations of chromophoric amino acids, including tyrosine, trypotophan, cystine, and cysteine. The extension of this approach to enzyme types involved with replication will be initiated in new work, to obtain information about the stabilization of DNA damage induced by inhibition of repair processes. The photodynamic inactivation of enzymes is being studied with particular attention to the role of singlet oxygen and the influence of dye binding on the mechanism. Current work on hen lysozyme in the presence of eosin will be extended to additional enzymes, including ribonuclease, trypsin, and papain, and to other sensitizing dyes.